On wafers provided with patterns such as LSI, CMOS or power devices formed on a substrate made of a compound such as silicon or sapphire, many devices are formed and partitioned by division lines, called scribe streets or simply streets. When the devices separated lengthwise and widthwise by streets are singularized into individual pieces by dividing at the streets, dicing is carried out by a blade or irradiation with a laser beam. The dicing method by a laser beam is one of the effective methods.
When the dicing is carried out by a laser beam, the irradiation of the laser beam causes explosions at the spots absorbing the laser beam, and processing grooves are formed. The explosions blast substances (debris) by sublimation from the wafer, and the blasted substances adhere as foreign matter (debris) to the portions adjacent to the processing grooves formed on the streets on the wafer surface, and part of the debris also unwantedly adheres to the surface of the devices as well.
As a countermeasure to overcome the foregoing drawbacks of the prior art, there is a method to provide such surroundings that can prevent adherence of debris sublimed and scattered by explosion to the wafers. As such method, a method of spraying an assist gas on the wafer surface to scatter the debris and decomposing and gasifying the scattered debris by use of an environmental gas; a method of vacuum suction; a method of processing under reduced pressure, and the like have been conventionally known. However, these conventional methods require the installation and operation of apparatuses and working conditions thereof are complex. And, removal of debris has not been sufficiently achieved even with such methods, and improvements have been demanded.
Under such circumstances, a method has been employed in which the wafer surface having various devices formed thereon is coated and protected with a resin prior to the irradiation with a laser beam, and the above laser processing treatment is conducted, and then the protective film is removed.
In this method, a water-soluble resin solution is used as a protective film material, and the protective film material is applied to the wafer surface having devices formed thereon by a spin coater to protect the surface. Then, after the laser processing treatment, the debris and the protective film material are washed away (JP-A-2007-073670).
Further, it has been known that a small amount of a laser beam absorbent is incorporated into the protective film material, and after the laser processing treatment, removal of debris is conducted (JP-B-4571850).
The above methods have achieved substantial success in preventing the fixing of debris in the areas near the streets of the wafer. However, these methods are not enough for protection of the device chips since the scattered debris sometimes scatters to the central portion of the device chips away from the streets and fixes thereto, or may sometimes cause burning, scorching or the like by thermal change.
Particularly, for image sensors to be used for cameras, attachment of even a small amount of debris forms stains on the lenses on the surface of the device chips, causing deterioration of performance of the image sensors and an increase in rejection rate, and further improvements to deal with such drawbacks have been demanded.